Grinding and polishing of glass sheets



Jan. 19, 1960 R. TOUVAY GRINDING AND POLISHING OF GLASS SHEETS Filed May 29, 1956 IN V EN TOR. ROBERT TOUVAY WEIGHT OF CARBORUNDUM IN THE RUBBING TOOL Liam ma ATTORNEY S United States Patent GRINDING AND POLISHING OF GLASS SHEETS Robert Tonvay, Paris, France, assignor to Ste Anonyme dee Manufactures des Glaces et Produits Chimiques de sh-Gobain, Chauny et Cirey, Paris, France Application May 29, 1956, Serial No. "588,004 Claims priority, application France June 3,1955

8 Claims. (Cl. 51-283) The invention relates to the grinding and polishing of glass sheets. It is known that the operation of grinding and polishing glass sheets comprises a plurality of successive phases, which consist in abrading the surface of the glass by means of abrasives which become progressively finer and which are in suspension in a liquid. In the final phase, or the polishingproper, generally carried out by means of tools having an operative felt surface which is supplied with a suspension of red iron oxide, or polishing powder, the energy required is relatively high; with equal expenditure of energy, a thickness of glass very much smaller than that which is removed in the preceding phase, known as smoothing, is worn away in this phase of the working operation. The type of glass with which this invention is most useful is generally called flat glass in American industry.

The condition of the surface after smoothing is thus of great importance: it must be such that the thickness of glass to be removed in the subsequent polishing phase is reduced to a minimum.

The invention is concerned with the working phase preceding the polishing and consists in that while the glass surface is fed with the abrasives usual for this phase, a tool formed by hard grains distributed uniformly in a material serving as a binder is usd as the rubbing tool.

The applicant has observed that, under these conditions, the energy necessary for carrying out the polishing is reduced by considerable proportions.

This result can apparently be explained in the following manner:

The working phase preceding the polishing is usually carried out by interposing an abrasive in suspension in water between the glass surface and a tool of homogeneous and hard consistency, generally of cast iron.

It can be admitted that the movements of the grains carried along beneath the homogeneous and hard tool is a rolling movement. In the course of this movement, each grain digs a series of very small holes by indenting the glass surface, the depth of said holes being a function of the size of the grain. in addition, however, these indentations have the effect of producing a cracked sublayer, of approximately the same thickness, beneath the layer comprising the small holes. The result is that, during the polishing, after the polishers have abraded the external layer where the small holes are localised, they come in contact with the subjacent cracked layer, their action causes a flaking or chipping of this layer and the action must be continued until it disappears, the result of which is the necessity of wearing away a thickness of glass substantially double that which it would be necessary to abrade in order to cause the individual small holes to disappear.

in the process according to the invention, on the other hand, it can be assumed that the abrasive supplied to the glass surface is confined between the hard grains set in the material serving as binder and that, under these conditions, the abrasive is carried along by the tool without rolling. It then traces on the glass scratches of a depth substantially equal to that of the small holes which it causes in the ordinary process, but with this essential difference that as a result of this working method, without impact of the abrasive grains, no subjacent layer is formed.

This provides an explanation for the fact that in the process of the invention, since the polishing operation only has to remove the layer in which the scratches are formed, this operation is considerably reduced with respect to the prior processes.

A large number of plastic materials and certain soft metals are capable of being used as binders for providing such a rubbing tool.

The hard grains incorporated with the binder (Carborundum, emery, corundum, metallic grains, etc.) will have a hardness of the same order as or larger than that of the sand used for working glass, and a size in practice which is smaller than 15011.

The ratio between the hard grains and the binder plays a very important part. The best results are obtained when the proportion is 8 to 17% by volume of hard grains with respect to the total volume of the rubbing tool. A smaller proportion or the complete absence of hard grains would give a much smaller wear on the glass surface, the consequence of which would be an excessive increase in the duration of the working phase preceding the polishing. A larger proportion would also give a very small degree of wear. With a rubbing tool formed for the major part or exclusively of the hard grain material, there would be caused the formation of the cracked sublayer, the existence of which necessitates an increase in the polishing work.

The rubbing tool must have a hardness which is decidedly less than that of the cast iron hitherto generally used in the same working phases. In practice, its degree of hardness measured with the Shore A hardness meter at 25 C., after 30 seconds, should be between and 95.

Example 66 g. of polyvinyl chloride, 34 g. of tricresyl phosphate, 40 g. of Carborundum grains are introduced into a mixer and these substances are thoroughly mixed. The agglomeration is then carried out on the cylinders of a rolling mill heated at C., sheets with a thickness of approximately 3 mm. being then drawn therefrom. A certain number of these sheets are assembled by compression at 70 kg. per cm. at C. in order to form a plate of the desired thickness, and the rubbing tools are cut in this plate. The proportion by volume of Carborundum grains in the plate is 13% and the degree of hardness thereof at 25 C., after 30 seconds, is 86 when measured by the Shore A hardness meter.

:In a grinding apparatus comprising cast iron rubbing tools and using sand sludges as abrasive, the cast iron rubbing tools are replaced by rubbing tools prepared as in dicated above for working with the three last sludges, of which the coarsest grains have mean diameters smaller than 40, 32 and 251.4 respectively. The rubbing tools 7 working with sludges having a grain size of 40;/., were formed with Carborundum grains of a size of 10011.. The rubbing tools working with the 33 and 25 i sludges were formed by Carborundum grains of 50a.

The polishing period has been reduced by half.

The accompanying drawing shows the coeflicient of wear or abrasion obtained with the rubbing tools consisting of plasticised polyvinyl chloride and Carborundum according to the example indicated above, as a function of the percentage by weight of Carborundum in the rubbing tool, the abrasive employed being a sludge, of which the coarsest grains do not exceed 2511.. It will be seen that for each of these curves, there exists a very clear maximum for a content by weight of Carborundum between 20 and 35%, the coeflicient of wear rapidly becoming insignificant on either side of this maximum, in particular for a zero content of Carborundum, that is to say, for rubbing tools constituted exclusively of plasticised vinyl chloride without Carborundum.

What is claimed is:

1'. A method for surfacing glass sheets in order to produce polished glass sheets, which includes the step of submitting the glass sheet to a grinding phase preceding the polishing phase and carried out by rubbing means comprising a binder and grains at least as hard as conventional free abrasives which are distributed uniformly in the binder, and introducing conventional free abrasives between the means and the glass. 2. A method for surfacing glass sheets in view of obtain'ing polished glass sheets, which consists in submitting the glass sheet to a grinding phase preceding the polishing phase and carried out by rubbing means comprising a binder, and grains at least as hard as conventional free abrasives in a proportion of 8 to 17 percent of the total volume of the rubbing means, which are distributed uniformly in the binder, and introducing conventional free abrasives between the means and the glass.

3. The method of claim 2 in which the grains set in the softer material constitute about 8%17% of the total volume of the softer material and the set grains. 4. The method of claim 2 in which the set grains are not over about 150p. in size.

5. The method of claim '2 in which the material comprised of the set grains and softer material has a hardness after 30 sec. at 25' C. of 75-95 on the Shore A meter.

6. The method of claim 1 in which the size of grain in the free abrasive is on the order of 40; and the size of set grains is on the order of loop 7. The method of claim 2 in which the size of grain in the free abrasive is on the order of 33-25;. and that of the set grains is on the order of 50 1,.

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